Stroke, head injury and tumors commonly cause loss of peripheral vision in a condition called Hemianopia. The number of disabled stroke survivors in the United States is estimated to be more than 3 million annually. As many as one third of stroke survivors in rehabilitation have either Homonymous Hemianopia or Hemineglect.
Homonymous Hemianopia, which is considered to be a visual field defect, is the loss of half the visual field on one side in both eyes. This condition causes difficulties with general mobility (obstacle detection and navigation), as people with Homonymous Hemianopia are likely to walk into obstacles on the side of the field loss, such as furniture or objects on the floor. Many people afflicted with this condition avoid busy places for fear they will bump into other people or objects. People with Homonymous Hemianopia are prohibited from driving in 22 states (Peli E, Peli D, 2002). In many other states, they are discouraged from driving even when the laws do not prohibit driving (Tant M L et al., 2001). Accordingly, people with Homonymous Hemianopia may lose much of their independence.
Various devices such as mirrors, partially reflecting mirrors (beam splitters), prisms, and reversed telescopes have been considered and used for management of Hemianopic field defects (Fanning G J, 1972; Rossi P W et al., 1990). The purpose of such devices is to improve patients' awareness of obstacles on their blind side to improve their mobility. These devices may be classified as providing either field relocation (shifting) or field expansion. Field expansion is preferred, since the simultaneous field seen by the person is larger with the device than without. Field relocation only exchanges the position of the lost field relative to the environment. A number of devices provide field expansion, but none has proven entirely successful (Peli E, 2000).
For example, reversed telescopes increase the visual field but reduce visual acuity. These devices are cumbersome, heavy and unsightly when mounted in spectacles and rarely used for Hemianopia. Mirrors expand the field in one direction, but at the same time block vision in another. Mirrors also reverse the images, making interpretation of the visual field difficult. In addition, mirrors are cosmetically poor, and safe mounting of mirrors close to the eye is difficult. Beam splitters expand the field through diplopia (double vision), which can be very confusing, and as an additional problem, the reflected images are reversed. Beam splitters also have the same cosmetic and safety issues associated with mirrors.
Prism devices find numerous applications for Hemianopia, but only a few of these actually expand the field. The monocular prisms that have been used in the past, including a sector prism base out in front of one eye with the edge of prism placed at the pupil, or the round button used in the Gottlieb system, result in double vision. Devices employing monocular sector prisms truly expand the visual field when the patient moves his/her direction of gaze to within the field of the prism. As long as the patient's eyes are at a primary position of gaze or are directed away from the Hemianopic field, the monocular sector prism has no effect on the field of view (i.e. no field expansion). When gaze is directed into the field of the prism, field expansion results. However, it is accompanied by (central) diplopia and the confusing appearance of two different objects in the same perceived direction. Confusion in this case represents the intended beneficial effect, as it represents the appearance of an object that would be invisible without the prism. However, the central diplopia induced with the prism is unpleasant to the patient and may account for the lack of success in sustained use of monocular sector prisms by people with Homonymous Hemianopia (Gottlieb D D et al., 1996).
There are two schools of thought as to why patients generally discontinued use of these prisms after 2-3 months. Some practitioners point out that patients discontinue using them because their scanning ability increases. Some even claim that wearing these prisms results in a restoration of some of the lost visual field (Gottlieb D D et al., 1998.) However, no scientific proof of such scanning increase or restoration change has been provided. Other practitioners state that patient frustration and the difficulty of use are why the patients discard them. Thus, although a variety of image-shifting devices such as prisms and mirrors have been proposed as aids for hemianopic individuals, none has proven very effective.
One prior art method that was able to expand the visual field rather than relocate it, function in all positions of gaze, and avoid central diplopia included a monocular sector prism that is disposed in the peripheral visual field of the user (Peli E, 2000). The monocular sector prism can be placed in the superior or inferior peripheral field, or it may be placed simultaneously in both. The monocular sector prism is placed across the whole width of the lens and is effective at all lateral positions of gaze. For example, in this configuration, a prism of 40Δ shifts the image by about 20 degrees. The prism expands the field via peripheral diplopia. Peripheral diplopia is much more comfortable for the user than central diplopia since peripheral physiologic diplopia is a common feature of normal vision. The field expansion effect is unaltered by lateral eye and head movements over a wide range of movements into either side.
The purpose of a system of one or more peripheral prisms is to provide obstacle awareness within the blind field in areas that are important for general mobility. Studies of persons with Homonymous Hemianopia have concluded that the most important areas for obstacle awareness that are important for general mobility are the central visual area that includes both outer and inner central areas. The outer and inner central areas are defined as a ring with a 21-37 degree radius and a circle with a 21 degree radius respectively. Studies of mobility performance of patients with restricted peripheral fields resulting from retinitis pigmentosa also suggest that extension of peripheral field loss into the central area in a 20 degree radius and a 10 degree radius significantly impacts on mobility performance.
The visual field expansion that is produced with the current arrangement of peripheral prisms in the prior art only extends a limited amount into the outer central area of the blind hemi-field. For example, the lower prism in this system is currently fitted with the upper edge at a height that will project it to 4 meters away on the ground when viewing a fixation point at eye level. For a person having a height of 5′6″ this translates to an angle of about 23 degrees below the fixation point. Thus the effect of the lower prism, i.e. the field expansion, will only start below this point and does not reach into the inner central area. A slightly higher fitting position is possible but in any case the central area remains unaffected by the prisms
Thus, it would be desirable to provide a simple and inexpensive way of moving the area of field expansion of the blind hemi-field into areas that are important for general mobility, i.e., towards the inner central visual area, without having to alter the fitting positions of the upper and lower prism segments.